This invention relates to the oxychlorination of ethylene in a fixed bed catalytic process to produce chlorinated hydrocarbons, particularly 1,2-dichloroethane.
It is well known that hydrocarbons such as ethylene may be chlorinated by reacting them with hydrogen chloride and gases containing elemental oxygen, particularly air, in the presence of a catalyst at elevated temperatures and pressures. Such a process is generally termed an "oxychlorination" or a Deacon process and usually employs a catalyst comprising a chloride of a metal having at least two valences, generally on a porous refractory support. The most common catalyst for such processes comprises cupric chloride on a particulate material such as activated alumina, silica, alumina-silica, diatomaceous earth, etc. Activated alumina, in one or another of its various forms, is the most common support utilized. In addition, the catalyst may contain additives such as alkali metal chlorides, rare earth metal chlorides, and other metallic compounds which assist in promoting the desired reaction and/or inhibiting the progress of side reactions. Particularly, potassium chloride has been utilized as an additive to such a catalyst when it is desired to produce 1,2-dichloroethane from ethylene since potassium chloride is known to suppress the formation of ethyl chloride. The amount of potassium chloride used, however, is kept low as it also tends to decrease the activity of the catalyst towards the primary reaction.
In the conduct of such a fixed bed oxychlorination process one of the concerns is the control of the reaction temperature. The oxychlorination reaction itself is highly exothermic and, in addition, control of the reaction temperature is hampered by the fact that the catalyst bed itself has a low heat conductivity. As a result of these two factors, there is a danger of the formation of undesirably extraordinarily high localized temperature zones in the catalyst bed. Numerous expedients have been proposed in the art aimed at preventing or at least minimizing the existence of such exceptionally high localized temperatures. For example, it has been variously proposed to control temperature by adjusting the ratio of the reactants; by diluting the feed with an inert gas or an excess of one or more reactant gases; by utilizing a tubular reactor with controlled external cooling and/or tubes of varying diameters; by diluting the catalyst particles with inert particles; and by varying the particle size of the catalyst and/or inert particles.
Particularly when the reaction is conducted in tubular reactors, it is known in the art, for example as described in U.S. Pat. No. 3,184,515, that the problem of undesirably high localized temperatures does not exist throughout the entire reactor. This is a result of the nature of the oxychlorination reaction itself, which becomes progressively less vigorous in the direction of flow of the reaction mixture. At the inlet of the catalyst bed, the reaction proceeds rapidly and strongly and control of both the temperature and location of the hot spot (point of highest temperature) in the bed is important. However, as the reactants proceed through the bed, the reaction becomes somewhat less vigorous as oxygen is consumed. This is particularly the case when, as is known in the art, the oxychlorination process is carried out in a series of two or more catalytic reactors with the total air (or oxygen) feed being split between the several reactors. Thus, as the oxychlorination feed mixture reacts, oxygen is used up toward the outlet of each reactor and the concern of a runaway reaction or overly high localized temperature in this zone is less than in the portion of the reactor closer to the inlet.
One solution which has been proposed for accomplishing acceptable conversions and selectivity to dichloroethane as well as obtaining reasonable control of the reaction temperatures and formation of high localized temperatures is to dilute the catalyst with inert particles intermingled with the catalyst particles. The inert particles may consist of, for example, silica, alumina, graphite, glass beads, etc. In some processes, the proportions of catalyst to diluent in the oxychlorination reactor have been varied. At the inlet side of the reactor, it is desirable to have a less concentrated catalyst because of the danger of runaway reactions or high localized temperatures. However, further from the inlet, as the reaction proceeds and becomes less vigorous, these dangers are not quite so prominent and in fact, it would be desirable to have a more highly active catalyst to continue promoting the reaction as oxygen is consumed. Thus, it is common when using diluted catalysts to divide the bed into two or more zones, each zone containing a different ratio of catalyst to diluent, with the ratio of catalyst to diluent increasing toward the outlet end of the reactor. For example, in U.S. Pat. No. 3,184,515, a process is described (Example 1) using a diluted catalyst in which the reaction tube is divided into four zones, the first zone containing 7 volume percent catalyst and 93 volume percent graphite diluent, the second zone containing catalyst to diluent in a 15:85 volume percent ratio, the third containing a 40:60 mixture, and the fourth containing 100 percent catalyst. In another variation, a fairly highly active catalyst may be placed at the very inlet of the reactor in order to initiate the reaction, immediately followed by a much less active catalyst to prevent the formation of hot spots in the adjacent reaction zone.
The use of a diluted catalyst such as described in the preceding paragraphs, however, possesses several disadvantages. In the first place, it requires loading of the catalyst in several different reaction zones and thus, the formulation of several catalyst-diluent mixtures of different proportions. Of greater concern however, is the fact that the mixing of catalyst and diluent can result in a non-uniform mixture. There is, therefore, a likelihood of formation of undesirably high localized temperatures due to a concentration of catalyst particles in a particular section of the reaction zone if the mixing is not carried out to a sufficiently thorough degree. Additionally, in many cases the diluent particles are not of the same general size or shape as the catalyst particles. It has been proposed, for instance, to dilute cylindrical catalyst particles or spherical catalyst particles with diluent particles of a different shape or size. In such cases, the overall mixture does not provide a reasonably uniform surface to the reactants and the pressures and/or pressure drops occurring during the reaction may be other than advantageous. Even if the diluent particles possess the same relative shape or size as the catalyst particles, there is still the likelihood that the diluent and catalyst will not be satisfactorily mixed and hot spots can result, as well as the nuisance of having to mix up different catalyst-diluent mixtures for the different reaction zones.
Another solution which has been proposed has been to provide a catalyst, either with or without diluent particles, in which the particle size decreases from inlet to outlet. Such a concept is described in U.S. Pat. No. 3,699,178. However, such a practice, even if the catalyst is utilized without a diluent, requires careful manufacture to assure that the particle sizes are as desired and requires the manufacture of at least two, and very likely three or four different catalysts, in order to attain the objectives of this concept. In the alternative embodiment of this concept, in which the catalyst is intermingled with diluent particles, the disadvantages of utilizing diluent particles are added to those involving the preparation of different sized catalyst particles.
It is an object of the present invention to provide an oxychlorination process for the production of 1,2-dichloroethane from ethylene.
It is a further object of this invention to provide an oxychlorination process for the production of 1,2-dichloroethane from ethylene in which hot spot location and temperature can be readily controlled.
It is another object of the present invention to provide a process for the production of 1,2-dichloroethane by oxychlorination of ethylene at high hydrogen chloride conversion rates.
Another object of the present invention is to provide such a process in which a substantially uniform pressure drop can be maintained for a reasonably long period of time.
Yet another object of the present invention is to provide such a process in which selectivity of conversion of ethylene to 1,2-dichloroethane is acceptably high, and in which excesses of reactants such as ethylene and/or air can be kept to a minimum.
Still another object of the present invention is to provide such a process which can be carried out at high flow rates of reactants.
Another object of the present invention is to provide a system and process for the oxychlorination of ethylene which can be run using either air or oxygen as the oxygen-containing gas.
Yet another object of the present invention is to provide a new catalyst for use in conducting the oxychlorination of ethylene to 1,2-dichloroethane with the above advantages.